Improvements to xts time series visualization and subsetting

by Michael Weylandt for R project for statistical computing

Major extension to the plotting and analytic capabilities of the xts package with intent to provide a full time series suite around a single S3 class designed for high-performance time series work. Intended work includes major extensions to plot.xts to allow multi-column time series and block highlighting, implementation of major ts functions [ARIMA, Holt-Winters, StructTS] for irregular data. Possible work includes new data structures to allow multiple data types in a single xts/zoo object.